1. Technical Field
The field of the invention is mobile communications and although not limited thereto is disclosed in the context of the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA) of the Third Generation Partnership Project (3GPP). In that context, the invention is disclosed as related to mobility for high speed downlink packet access (HSDPA) and high speed uplink packet access (HSUPA), i.e., HS-DSCH/E-DCH Serving Cell Change which is possible currently only with the RL Reconfiguration procedure over the Radio Network Subsystem interfaces (Iub/Iur), but the invention is not limited to that specific technical environment.
2. Discussion of Related Art
Referring to FIG. 1, the Universal Mobile Telecommunications System (UMTS) packet network architecture includes the major architectural elements of user equipment (UE), UMTS Terrestrial Radio Access Network (UTRAN), and core network (CN). The UE is interfaced to the UTRAN over a radio (Uu) interface, while the UTRAN interfaces to the core network over a (wired) Iu interface.
FIG. 2 shows some further details of the architecture, particularly the UTRAN. The UTRAN includes multiple Radio Network Subsystems (RNSs), each of which contains at least one Radio Network Controller (RNC). Each RNC may be connected to multiple Nodes B which are the 3GPP counterparts to GSM base stations (a second generation Radio Access Technology (RAT)). Each Node B may be in radio contact with multiple UEs via the radio interface (Uu) shown in FIG. 1. A given UE may be in radio contact with multiple Nodes B even if one or more of the Nodes B are connected to different RNCs. For instance a UE1 in FIG. 2 may be in radio contact with Node B 2 of RNS 1 and Node B 3 of RNS 2 where Node B 2 and Node B 3 are neighboring Nodes B. The RNCs of different RNSs may be connected by an Iur interface which allows mobile UEs to stay in contact with both RNCs while traversing from a cell belonging to a Node B of one RNC to a cell belonging to a Node B of another RNC. One of the RNCs will act as the “serving” or “controlling” RNC (SRNC or CRNC) while the other will act as a “drift” RNC (DRNC). A chain of such drift RNCs can even be established to extend from a given SRNC. The multiple Nodes B will typically be neighboring Nodes B in the sense that each will be in control of neighboring cells. The mobile UEs are able to traverse the neighboring cells without having to re-establish a connection with a new Node B because either the Nodes B are connected to a same RNC or, if they are connected to different RNCs, the RNCs are connected to each other. During such movements of a UE, it is sometimes required that radio links be added and abandoned so that the UE can always maintain at least one radio link to the UTRAN. This is called soft-handover (SHO).
The handover function is based on radio measurements, and it is used to maintain the quality of service requested by the core network. The handover strategy employed by the network for radio link control determines the handover decision that will be made based on the measurement results reported by the UE/RNC and various parameters set for each cell. Network-directed handover might also occur for reasons other than radio link control, e.g., to control traffic distribution between cells. A given network operator determines the exact handover strategies, but possible types include 3G-3G handover, FDD soft/softer handover, FDD inter-frequency hard handover, FDD/TDD handover, TDD/FDD handover, TDD/TDD handover, 3G-2G handover and vice versa. Causes for initiation of a handover process are many, including uplink quality, uplink signal measurements, downlink, downlink signal measurements, distance, change of service, better cell, O&M intervention, directed retry, traffic, pre-emption, etc
Regarding soft handover, it is described in 3G TR 25.922 v.3.1.0 (2000-03) at Chapter 5.1.4. There, soft handover is described as a handover in which the mobile station starts communication with a new Node B on a same carrier frequency, or sector of the same site (softer handover) performing at most a change of code. With reference to soft handover, the “active set” is defined as the set of Nodes B the UE is simultaneously connected to, i.e., the UTRA cells currently assigning a downlink DPCH to the UE constitute the active set. The soft handover procedure is composed of a number of single functions: (1) measurements, (2) filtering of measurements, (3) report of measurements results, (4) the soft handover algorithm, and (5) execution of handover.
The measurement of the monitored cells filtered in a suitable way trigger the reporting events that constitute the basic input of the soft handover algorithm. The definition of “active set”, “monitored set”, as well as the description of all reporting events, is given in TS 25.331, V6.6.0 (2005-06) “Radio Resource Control (RRC); Protocol Specification (Release 6).” Based on the measurements of the set of cells monitored, the soft handover function evaluates if any Node B should be added to (radio link addition), removed from (radio link removal), or replaced in (combined radio link addition and removal) the active set; performing then what is known as “active set update” procedure. An example of a soft handover algorithm, as well as its execution, is shown in Chapter 5.1.4.2 and 5.1.4.3 of 3G TR 25.922, as well as Annex C thereof, which shows a flowchart of a soft handover algorithm.
3GPP TS 25.303 v.4.0.0 (2001-03) shows radio link addition for FDD in Chapter 6.4.4 thereof. As suggested above, radio link addition is triggered in the network RRC layer by measurement reports sent by the UE. The network RRC first configures the new radio link on the physical layer. Transmission and reception begin immediately. The network RRC then sends an RRC active set update message to the UE RRC. The UE RRC configures Layer 1 to begin reception. After confirmation from the physical layer in UE, an active set update complete message is sent to the RNC-RRC.
It has been agreed within the 3GPP to add a shared channel, i.e., the so-called High Speed Downlink Packet Access (HSDPA) concept to the UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network (UTRAN) architecture, see 3GPP TS 25.308 v6.3.0 (2004-12). The basic idea behind the HSDPA is to offer a shared high speed downlink (transport) channel (called HS-DSCH, for high-speed downlink shared channel) for use in communicating packet data to a UE (user equipment) device. As with the current DSCH, every UE device to which data can be transmitted on the HS-DSCH has an associated dedicated physical channel (DPCH). The DPCH is used to carry power control commands for the associated uplink, and if needed, other services, such as circuit-switched voice. The HS-DSCH offers a higher data rate and a fast retransmission mechanism, namely the HARQ (Hybrid Automatic Repeat Request) mechanism, provided by Node B. In pre-release 5 implementations, the only official shared channel in 3GPP in downlink was DSCH for which retransmission was to be always provided by the RLC (Radio Link Control) in the RNC (Radio Network Controller) of UTRAN, which was a relatively slow retransmission mechanism. However, since Release 5, DSCH has been deleted from the specifications due to lack of a need therefor. A similar dedicated channel (E-DCH) has been agreed for High Speed Uplink Packet Access (HSUPA). Thus, a similar enhancement is contemplated for the uplink as set forth in 3GPP TS 25.309 v6.3.0 (2005-06) “FDD Enhanced Uplink; Overall description; Stage 2 (Release 6).” HS-DSCH mobility procedures and E-DCH Specific Scenarios are outlined in sections 7.10 and 7.20, respectively, of 3GPP: “Technical Specification Group RAN; TR 25.931 v6.2.0 (2005-06); UTRAN functions, examples on signaling procedures (Release 6).” Details of an “Active Set Update (ASU) with the HS-DSCH” has been described in co-pending U.S. provisional application Ser. No. 60/614,562 filed Sep. 29, 2004 (now U.S. patent application Ser. No. 11/237,643 which is incorporated by reference for background.
RAN2 has discussed in the contribution R2-042103 (in RAN2#44) about the enhancement to Serving HS-DSCH cell change when there is a simultaneous need to update the Active Set. RAN2 has then discussed the proposal to include the enhancement into the RAN2 RRC specification in contribution R2-050115 (in RAN2#45bis), and then agreed the CR to RRC specification in contribution R2-051203 (in RAN2#46bis).
The purpose of the Enhancement is that when the need for Active Set Update (ASU) and HS-DSCH cell change occurs simultaneously, the SRNC can avoid executing two separate procedures; Active Set Update and e.g. Physical Channel Reconfiguration to the UE. This might lead to a service break, which will happen because of the parallel procedure principle, which prohibits the simultaneous parallel procedures.
RAN2 has agreed the importance of the enhancement, and agreed to include the HS-DSCH Serving Cell change functionality into the RRC: Active Set Update procedure. This will therefore lead to faster ASU and HS-DSCH Cell change, which means a smaller transmission break or a break can be fully avoided.
For serving cell change and branch addition/deletion, three RNSAP/NBAP procedures (RL Addition/Deletion, Synchronised Radio Link Reconfiguration and Radio Link Reconfiguration Commit) must be executed according to the current specifications.